THE ARTHROPODA OF THE DEEP SEA
The deep-sea fauna seems to be particularly rich in marine Arthropoda,
many curious and interesting forms being brought up with almost every
haul of the dredge. The Arthropoda, too, being very highly organised
animals, afford interesting and instructive examples of the effect of
abysmal life in the modification of the sense organs and the production
of varieties specially modified for the conditions of the struggle for
existence in their strange habitat.
Concerning the groups of Ostracoda and Copepoda it may be said that the
evidence is not yet conclusive that they include any truly deep-sea
species. The largest known Ostracod, measuring somewhat more than an
inch in length and probably allied to the genus _Crossophorus_, has
quite recently been captured by Professor Agassiz at depths of less than
200 fathoms, but he could obtain no evidence that it descended into much
deeper water than this.
Mr. Brady, in writing the report of the ‘Challenger’ Ostracoda, came to
the conclusion that they do exist in very limited numbers in the most
profound depths of the sea; but it is nevertheless quite possible that
all the Ostracods brought on deck by the trawl or dredge were really
captured either on the way down or on the way up, and are, strictly
speaking, pelagic in habit.
Similar caution must be taken in dealing with the Copepoda, an order of
Crustacea that is essentially pelagic in habit. The only species that
has been regarded as undoubtedly abysmal is _Pontostratiotes
abyssicola_, a form whose carapace and antennæ are armed with
exceedingly long and strongly toothed spines, and was found in the mud
brought up by the trawl from a depth of 2,200 fathoms.
_Calamus princeps_, the largest species of its genus of a deep reddish
brown colour, may also belong to the fauna of the deep sea, but we have
less evidence concerning the habitat of _Hemicalamus aculeatus_,
_Phyllopus bidentatus_, and some of the Euchætæ.
The Amphipoda seem to be but poorly represented in the fauna of the
abyss; in fact it may be considered to be still an open question whether
any Amphipods habitually live in very deep water.
In the reports on the ‘Challenger’ Amphipoda, the Rev. T. R. Stebbing
states that thirty-one specimens are known to come from great depths,
but it would be more correct to say that these specimens were found in
the dredges and trawls that had been lowered into the great depths. It
should be noticed, however, that some of these specimens do show
characters that suggest, at any rate, that they come from deep water.
Thus the genus _Lanceola_, for example, is characterised by the
smallness of the eyes and a soft membranous integument, while _Cystisoma
spinosum_, found in a dredge that had been at work at a depth of over a
thousand fathoms, has very large eyes.
In his report on the Crustacea of the ‘Norske Nord-havns’ expedition,
Professor Sars gives a full description of many species of Amphipoda
brought by the dredge from depths of over 1,000 fathoms, and nearly all
of these were found to be quite blind.
The form that seems to be most peculiar to the great depths of the
Northern Ocean is _Harpinia abyssi_. It was found at no less than
fifteen different stations at depths ranging from 350 to 2,215 fathoms,
and is characterised by its large size and the total absence of eyes.
Another point that should be considered in coming to any conclusion on
the supposed habitat of such forms, is the similarity or dissimilarity
of widely distributed species.
I have had occasion to point out in a previous chapter the general
similarity of the abysmal fauna all over the world, a very striking
phenomenon, commented on by almost every naturalist who has had a wide
experience of this kind of investigation.
Among the Amphipoda we have a very striking example of this. The species
_Orchomene musculosus_ was taken by the ‘Challenger’ off the southern
part of Japan at a depth of 2,425 fathoms, the bottom being red clay and
the temperature 35·5° Fahr. The species _Orchomene abyssorum_ was taken
off the east coast of Buenos Ayres at a depth of 1,900 fathoms, the
bottom being blue mud and the temperature 33·1° Fahr. To the description
of this last-named species Mr. Stebbing adds, ‘had this species been
taken within reasonable distance of _O. musculosus_, the resemblance is
so great that one might have been tempted to disregard the points of
difference as due to some other cause than difference of species.’
Such a striking similarity between two species living so far apart from
one another may, when we take into consideration the depth, the
character of the bottom, and the temperature from which they are
supposed to have been dredged, be taken to support very strongly the
view that these species are really abysmal in habit.
Among the Isopoda we have several very characteristic forms—no fewer
than nine distinct genera peculiar to the abysmal zone have been
described by Beddard—and of these two, _Bathynomus_ and _Anuropus_, are
to be regarded as types of sub-families. They seem to be very unevenly
distributed over the floor of the ocean, some regions, such as the whole
of the Central and Southern Atlantic and the Central and Western
Pacific, produce none; whilst the waters of the east coast of New
Zealand, the Crozets, and others, produce a great many varieties. Many
of the deep-sea Isopoda exhibit characters that are usually associated
with the bathybial life. Thus, according to Beddard, thirty-four of the
deep-sea species are totally blind, and eighteen have well-developed
eyes. In four species there are eyes which are evidently degenerating.
If we compare, for instance, the structure of the eye of _Serolis
schythei_, a species found in shallow water ranging from 4 to 70
fathoms, with the eyes of _Serolis bromleyana_, a species living in deep
water ranging from 400 to 1,975 fathoms, we cannot fail to see that the
latter are undergoing a process of degeneration; the retinulæ and
pigment being absent, and nothing left of the complicated structure of
the Isopod eye but the remnants of the crystalline cones and corneal
facets (see figs. 4 and 5, p. 74).
Taking the genus _Serolis_ alone, it has been said ‘that in all the
shallow-water forms the eye is relatively small but very conspicuous
from the abundant deposition of pigment; in all the deep-sea forms, with
the exception of _S. gracilis_, where the eye seems to be disappearing,
it is relatively larger but not so conspicuous, owing to the fact that
little or no pigment is present.’
In many groups of animals it has been shown that some of the deep-sea
species are relatively much larger in size than the shallow-water
species, and that others, more rarely, are much smaller, the abysmal
fauna reminding us in this respect of the characters of the alpine
flora.
The Isopoda show many examples of this largeness in size, thus
_Bathynomus giganteus_, dredged by Professor Agassiz off the Tortugas at
a depth of over 900 fathoms, reaches the enormous size, for an Isopod,
of 9 inches (fig. 15). _Stenetrium haswelli_, again, is larger than any
of the shallow-water species of the genus, and the same remark applies
to the deep-sea species of the genus _Ichnosoma_, while _Iolanthe
acanthonotus_, from a depth of nearly 2,000 fathoms, is considerably
larger than most of the shallow-water Asellidæ.
There is another very common character of deep-sea Crustacea that is
also well exemplified in the group of the Isopods, and that is the
extraordinary length and number of the spines covering the body.
I have already referred to this character in the supposed deep-sea
Copepod _Pontostratiotes abyssicola_, and I shall have again to refer to
it in treating of the Decapoda and other groups of the Crustacea.
Besides its enormous size _Bathynomus_ possesses some other characters
that may be correlated with its deep-sea environment. The respiratory
organs are quite different from those of other Isopods; instead of being
borne by the abdominal appendages, they are in the form of branched
outgrowths from the body-wall containing numerous blood-lacunæ, and the
appendages simply act as opercula to cover and protect them. The eyes of
the Bathynomus too are remarkably well developed, each one bearing 4,000
facets, and they are directed not dorsally as in the Cymothoadæ, but
ventrally. The cause of these curious modifications of structure in
Bathynomus is by no means clear, but it is quite probable that they are
connected with the conditions of pressure and light in the deep sea. It
is a remarkable fact that the other deep-sea Isopods do not exhibit
precisely these modifications, and it might be supposed that the same
causes would produce the same or similar effects on the structure of
animals belonging to the same order. That is perfectly true, but we
cannot yet determine how long ago any one species has taken to a
deep-sea life, or what length of time, in other words, these conditions
have been at work in modifying the structure of the organism. A recent
immigrant into the abyss will naturally exhibit closer affinities with
its shallow-water allies than those that have dwelt in the region since
secondary or tertiary times. If we take this into consideration we
should expect to find considerable differences occurring between
deep-sea species of the same order, which is precisely what we do find.
[Illustration: FIG. 15.—_Bathynomus giganteus._ From a depth of 1,740
metres. (From Filhol.)]
Concerning the Cirripedia, that curious group of profoundly modified
Crustacea that includes the barnacles and acorn shells, Dr. Hoek writes
in the ‘Challenger’ monograph:—
‘Though unquestionably by far the greater part of the known Cirripedia
are shallow-water species, and though some of the species are capable of
living at a considerable variety of depths, as, for instance,
_Scalpellum stroemii_, yet it must be granted that the number of true
deep-sea species of Cirripedia is very considerable.’ Only two genera,
however, occur in depths of over 1,000 fathoms, and these—_Scalpellum_
and _Verruca_—occur also as fossils in secondary and tertiary deposits.
The oldest of all fossil cirripedes, however, namely, _Pollicipes_,
never occurs, at the present day, in deep water, but is purely littoral
or neritic in habit. But what is perhaps more interesting still is the
fact, that, when we come to compare the living and the fossil species,
we find that in the one genus (_Scalpellum_) the deep-sea forms have
preserved the more archaic characters, and in the other (_Pollicipes_)
the shallow-water forms.
Here then we are presented with a veritable puzzle for which we can at
present frame no manner of answer. Pollicipes on the one hand—like
Lingula among the brachiopods—has been able to maintain itself almost
unchanged amid the tremendous struggle for life of the shallow water of
the tropics ever since the Lower Oolite epoch; while Scalpellum, on the
other hand, has either become profoundly modified, or been driven into
the abysmal depths of the ocean.
The group of the Thoracostraca, or stalk-eyed Crustacea, including
lobsters, crabs, hermit crabs, prawns, and shrimps, is well represented
in the deep sea. Most of them are characterised by being quite blind (in
many cases even the eye-stalks are obliterated), by being protected with
a dense covering of spines, by the thinness of their shells, and by
their bright red or carmine colour.
The order Stomatopoda is almost entirely confined to the shallow waters
of the tropical or temperate shores. Not a single species is known to
inhabit the deep sea, and only a very few specimens have been captured
in more than a few fathoms of water.
The Schizopoda, however, present us with many curious abysmal forms.
Most of the genera of this order belong to the pelagic plankton, and
many of them are known to possess the power of emitting a very strong
phosphorescent light. Several genera, however, such as _Gnathophausia_,
_Chlaraspis_, _Eucopia_, _Bentheuphausia_, &c., never seem to leave the
great depths of the ocean, and nearly all of these genera are
distinguished by being quite blind or possessing very much reduced or
rudimentary eyes.
[Illustration: FIG. 16.—_Euphausia latifrons_, from the surface of the
sea. (After Sars.)]
[Illustration: FIG. 17.—_Bentheuphausia amblyops_, from 1,000 fathoms.
(After Sars.)]
If we compare, for example, the pelagic _Euphausia latifrons_ (fig. 16)
with the nearly allied but abysmal _Bentheuphausia amblyops_ (fig. 17),
the difference in this respect between a Schizopod living in the
sunlight and one living in the darkness of the deep-sea is very
apparent.
The pelagic Schizopoda are usually quite pale and transparent; the
deep-sea forms on the other hand are frequently if not invariably of a
bright red colour, as is the case with many other deep-sea Crustacea to
which reference will be made later on.
Passing on to the group of the Decapoda, we find that the most
interesting of all the abysmal cray-fish is the family of the Eryonidæ;
indeed, in some respects the discovery of these curious forms may be
reckoned among the most valuable results of the ‘Challenger’ Expedition.
They are characterised by the dorsal depression of the anterior part of
the cephalothorax, the absence of a rostrum, and the absence or very
rudimentary condition of the eyes (fig. 18).
Their nearest relations seem to be certain genera of Crustacea that are
found in jurassic strata, in the lias, and more particularly in the
lithographic slates of Solenhofen.
They have a very wide bathymetrical range extending from a depth of 250
fathoms (_Polycheles crucifera_) to a depth of 2,000 fathoms
(_Willemoesia_).
[Illustration: FIG. 18.—_Polycheles baccata_, one of the Eryonidæ. The
eyes and eye-stalks are absent, and the margin and sides of the carapace
armed with spines. (After Spence Bate.)]
But there are many other curious forms of the macrurous crustacea that
deserve a passing mention. The graceful _Nematocarcinus gracilipes_,
distinguished by the extraordinary length of the antennæ and last four
pairs of legs, these appendages being three or four times the length of
the body, is by no means rarely met with in depths of over 400 fathoms.
The genus _Glyphus_ captured by the ‘Talisman’ is remarkable for the
development of a peculiar pouchlike arrangement on the abdomen for the
protection of the larvæ during the younger stages of their existence.
The proof of the existence of a peculiar cray-fish, _Thaumastocheles
zaleuca_, at a depth of 450 fathoms, was one of the most important
contributions to carcinology made by the ‘Challenger’ Expedition. The
chelæ of this remarkable form are of great but unequal length and armed
with long tooth-like spines giving it an appearance not unlike that of
the jaws of some carnivorous fish. The shell is soft and the abdomen
broad and flattened. There are no eyes nor even eye-stalks, but ‘in
front of the carapace,’ as Sir Wyville Thomson remarks, ‘between the
anterior and upper edge and the insertions of the antennæ, in the
position of the eyes in such forms as _Astacus fluviatilis_, there are
two round vacant spaces, which look as if the eye-stalks and eyes had
been carefully extirpated and the space they occupied closed with a
chitinous membrane.’ The deep-sea prawn, _Psalidopus_, recently taken in
500 fathoms of water by the ‘Investigator,’ affords us an example of a
common bathybial character, the whole body being covered with an
extraordinary array of sharp needle-like spines.
Among the crabs many curious forms have been found in deep water
extending down to depths of over 2,500 fathoms. They are nearly all
characterised by blindness and a remarkable development of tooth-like
spines covering the carapace and limbs.
The remarkable _Lithodes ferox_, from a depth of from 450 to 800
fathoms, is perhaps the most perfectly armed crab—in the way of
spines—that exists. Every part of the body and limbs is so covered with
spines that one has to be extremely careful in handling even a dead
specimen.
This is only one of the many examples that might be given to illustrate
this curious feature of the deep-sea Crustacea. Among the crabs alone we
have such forms as _Galathodes Antonii_, _Pachygaster formosus_,
_Dicranodromia mahyeuxii_ covered with a fierce armature of spines or
bristles; but there are nevertheless some species in which this
character is not particularly noticeable, and in these we usually find
some other protection against their enemies. An interesting example of
this has been described by A. Agassiz in a crab allied to the Maiadæ,
‘in which the dorsal face appears like a bit of muddy area covered by
corals, with a huge white arm resembling a fragment of an Isis-like
gorgonian.’ It is evident that this is a case in which the animal is
protected by its resemblance to the surroundings.
The hermit crabs of the abyss, too, are not usually characterised by any
very great development of spines. They find their protection in the
shells they inhabit. Some of the deep-sea hermit crabs carry about with
them on their shells a sea anemone, as we find to be frequently the case
among the shallow-water species. _Pagurus abyssorum_, from a depth of
3,000 fathoms, is an example of this.
In cases where there is a scarcity of gasteropod shells the hermit crabs
are obliged to find some other form of protection for their bodies. The
‘Blake’ found in the West Indies a hermit crab that had formed for
itself a case of tightly compressed sand, and another curious form,
named _Xylopagurus rectus_, makes its home in pieces of bamboo or in the
holes in lumps of water-logged wood.
The last group of the Arthropoda we need refer to is that of the
Pycnogonida, those curious creatures seemingly made up entirely of legs,
and by some naturalists considered to be related to the Crustacea and by
others to the scorpions and spiders.
Like the Brachiopoda the Pycnogonida are not usually found in greater
depths than 500 fathoms. Out of the twenty-seven known genera, only five
extend into the abyss, and not one of these can be called a true
deep-sea genus.
There are three genera, _Nymphon_, _Collosendeis_, and _Phoxichilidium_,
that show a very wide distribution over the floor of the ocean, and are
capable of existing at the greatest depths, and of these the species of
the genus Nymphon have a truly remarkable range extending from the shore
to a depth of 2,225 fathoms.
‘As a rule,’ says Hoek, ‘the deep-sea species are slender, the legs very
long and brittle, and the surface of the body smooth.’ They have
further, either no eyes at all or rudimentary eyes without pigment, and
in many cases—as, for example, _Collosendeis_—they are distinguished for
reaching to a gigantic size compared with their shallow-water relatives.
The Tunicata is the group of animals that includes all those curious
vegetable-like organisms found upon our coasts that are familiarly known
as sea-squirts, or Ascidians, besides the salps, pyrosomas, and the
microscopic appendicularias of the pelagic plankton.
[Illustration: FIG. 19.—_Collosendeis arcuatus_, from a depth of 1,500
metres. (After Filhol.)]
Notwithstanding the apparent simplicity of their adult structure,
naturalists are now agreed that they must be removed from the Mollusca,
with which they have hitherto been most frequently associated, and
placed in the group of the Vertebrata. It is the study of embryology
that has led to this unexpected conclusion, for we find, when we study
the larval forms, that they possess both a notochord and gill-slits, two
features that are characteristic of the group of the Vertebrata.
The species of the group Perennichordata, which includes all those
Tunicates that possess a notochord persistent through life, are chiefly
pelagic in habit, the little creatures, rarely more than two or three
millimetres in length, swimming or drifting about with the sagittas,
copepods, ctenophores, and medusæ that compose the pelagic plankton. Fol
has recently described a gigantic form belonging to this group, reaching
a size of thirty millimetres in length, called _Megalocercus abyssorum_,
which he dredged from a depth of 492 fathoms; and other species have
been recorded down to a depth of 710 fathoms in the Mediterranean Sea.
Among the simple Ascidians we find no family that is peculiar to deep
water; but the Cynthiidæ and Ascidiidæ both contain genera that are
abysmal, and the Molgulidæ have one species, _Molgula pyriformis_, that
extends into the abysmal zone to a depth of 600 fathoms.
In the genus _Culeolus_ and in _Fungulus cinereus_ and _Bathyoncus_, all
deep-water Ascidians, there is a very curious modification of the
branchial sac, the stigmata being apparently not formed, in consequence
of the suppression of the fine inter-stigmatic vessels. This peculiar
feature is only found in the deep-sea simple Ascidians and, as we shall
see presently, in one species of the deep sea compound Ascidians, but it
is not apparently an essential character of those living in the abysmal
zone, notwithstanding the fact that it is found in such widely separated
genera; for _Corynascidia_, _Abyssascidia_, and _Hypobythius_, living in
depths lying between 2,000 and 3,000 fathoms below the surface, have
branchial sacs of the ordinary type. Professor Herdman is of opinion
that this simple form of branchial sac is not a primitive form, but most
probably a modification of a more complicated type.
In _Culeolus Murrayi_ there is a remarkably abundant supply of
blood-vessels to the tunic, and these send special branches to a number
of small papilliform processes on its outer surface. This system of
highly vascular processes probably constitutes, as Professor Herdman
suggests, an additional or complementary respiratory apparatus. All
these modifications of the branchial system are of particular interest,
for we find so many instances of a similar kind among the inhabitants of
very deep water. I need only refer here to the modifications of this
system in the Isopod _Bathynomus_ already referred to (p. 129), and to
the reduction in the number of the gills of many of the deep-sea fishes.
Why there should be such modifications is a question upon which the
physical and natural history investigations of the conditions of life in
the great depths of the ocean at present throw no light.
In a previous chapter I have referred to the fact that many of the
bathybial animals are characterised by being stalked. Among the simple
Ascidia we find many examples of stalked kinds living in deep water,
such as _Culeolus_ and _Fungulus_, but also several exceptions, such as
_Bathyoncus_, _Styela bythii_, and _Abyssascidia_, that are sessile. It
is a noteworthy fact, however, that the genus that has the most deep-sea
species—namely, _Culeolus_—is a genus that is provided with a very long
stalk. Furthermore, the only known stalked forms of the very large
family Ascidiidæ are the abysmal genera _Corynascidia_ and
_Hypobythius_.
[Illustration: FIG. 20.—_Hypobythius calycodes._ G, nerve ganglion; H,
heart; M, the position of the atriopore. The large opening on the upper
side is the mouth. (From a drawing by Professor Moseley in Herdman’s
‘Tunicata of the “Challenger” Expedition.’)]
The most remarkable character of the genus _Hypobythius_ is the simple
condition of its branchial sac, reminding one of the structure of this
organ in the shallow-water genus _Clavelina_. ‘There are no folds and
there are no internal bars,’ to quote the description given by Professor
Herdman; ‘only a single system of vessels can be recognised, branching
and anastomosing so as to form a close network, the small rounded meshes
of which are the stigmata. The tentacles and dorsal lamina cannot be
made out.’
Among the compound Ascidians only four families extend into the abysmal
zone, namely, the Botryllidæ, Polyclinidæ, Didemnidæ, and Cœlocormidæ,
and of these only one species, _Pharyngodictyon mirabile_, of the family
Polyclinidæ, extends into water of greater depth than 1,000 fathoms. In
_Pharyngodictyon_ we find the same curious simplification of the
branchial sac that we have just referred to in the genera of simple
Ascidians, _Culeolus_, _Fungulus_, and _Bathyoncus_. _Cœlocormus
Huxleyi_ from a depth of 600 fathoms is a very peculiar form and the
type of a separate family, the Cœlocormidæ.
The free-swimming Tunicata included in the group _Ascidiæ salpiformes_,
which contains the genus _Pyrosoma_, and the order Thaliacea containing
the salps, are in all probability mainly confined to the surface waters.
A few specimens of _Pyrosoma_ were captured by the ‘Challenger’ dredges
which came up from very deep water, but it is doubtful at what point in
the journey to the surface the specimens entered the net.
The most remarkable form of free-swimming Tunicate that has come to
light is _Octacnemus bythius_, a form that is probably allied to
_Salpa_. It was found twice, once in the dredge that came from a depth
of 1,070 fathoms, and once from 2,160 fathoms. The tunic of the animal
is gelatinous and hyaline, but the most important feature it possesses
is an imperforate membrane separating the branchial sac from the
peribranchial cavity. Octacnemus, in other words, possesses no true
stigmata, these structures being represented only by little pits in the
walls of the branchial sac. This curious and extremely interesting
modification of the respiratory organs points very strongly to the
conclusion that Octacnemus is truly a deep-sea animal.